Prevent out of bounds pages from returning a next link.

Also makes sure the prev link is a real page.

Self link is unmodified, it will return a link to the current page
even if it doesn't exist. Seemed to best conform to JSON:API spec.

CHANGELOG.md

@@ -16,6 +16,7 @@ Features:
 Fixes:
 
 - [#2022](https://github.com/rails-api/active_model_serializers/pull/2022) Mutation of ActiveModelSerializers::Model now changes the attributes. Originally in [#1984](https://github.com/rails-api/active_model_serializers/pull/1984). (@bf4)
+- [#2171](https://github.com/rails-api/active_model_serializers/pull/2171) Prevent the `next` link from getting populated in JSON:API when an out of bounds page is requested. (@bcaplan)
 - [#2200](https://github.com/rails-api/active_model_serializers/pull/2200) Fix deserialization of polymorphic relationships. (@dennis95stumm)
 
 Misc:

lib/active_model_serializers/adapter/json_api/pagination_links.rb

@@ -53,11 +53,16 @@ def last_page_url
 
         def prev_page_url
           return nil if collection.current_page == FIRST_PAGE
-          url_for_page(collection.current_page - FIRST_PAGE)
+
+          if collection.current_page > collection.total_pages
+            url_for_page(collection.total_pages)
+          else
+            url_for_page(collection.current_page - FIRST_PAGE)
+          end
         end
 
         def next_page_url
-          return nil if collection.total_pages == 0 || collection.current_page == collection.total_pages
+          return nil if collection.total_pages == 0 || collection.current_page >= collection.total_pages
           url_for_page(collection.next_page)
         end
 

test/adapter/json_api/pagination_links_test.rb

@@ -88,6 +88,18 @@ def last_page_links
           }
         end
 
+        def out_of_bounds_page_links
+          {
+            links: {
+              self: "#{URI}?page%5Bnumber%5D=4&page%5Bsize%5D=2",
+              first: "#{URI}?page%5Bnumber%5D=1&page%5Bsize%5D=2",
+              prev: "#{URI}?page%5Bnumber%5D=3&page%5Bsize%5D=2",
+              next: nil,
+              last: "#{URI}?page%5Bnumber%5D=3&page%5Bsize%5D=2"
+            }
+          }
+        end
+
         def expected_response_when_unpaginatable
           data
         end
@@ -120,6 +132,13 @@ def expected_response_with_last_page_pagination_links
           end
         end
 
+        def expected_response_with_out_of_bounds_page_pagination_links
+          {}.tap do |hash|
+            hash[:data] = []
+            hash.merge! out_of_bounds_page_links
+          end
+        end
+
         def expected_response_with_empty_collection_pagination_links
           {}.tap do |hash|
             hash[:data] = []
@@ -174,6 +193,18 @@ def test_last_page_pagination_links_using_will_paginate
           assert_equal expected_response_with_last_page_pagination_links, adapter.serializable_hash
         end
 
+        def test_out_of_bounds_page_pagination_links_using_kaminari
+          adapter = load_adapter(using_kaminari(4), mock_request)
+
+          assert_equal expected_response_with_out_of_bounds_page_pagination_links, adapter.serializable_hash
+        end
+
+        def test_out_of_bounds_page_pagination_links_using_will_paginate
+          adapter = load_adapter(using_will_paginate(4), mock_request)
+
+          assert_equal expected_response_with_out_of_bounds_page_pagination_links, adapter.serializable_hash
+        end
+
         def test_not_showing_pagination_links
           adapter = load_adapter(@array, mock_request)